1. Field of the Invention
The present invention relates to a belt device capable of protecting a belt from stretch ascribable to thermal expansion and therefore from the variation of moving speed, and an image forming apparatus using the same.
2. Description of the Background Art
A copier, printer, facsimile apparatus or similar image forming apparatus is constructed to develop a latent image formed on a photoconductive drum or similar image carrier with toner and transfer the resulting toner image to a sheet or recording medium. A monochromatic toner image, for example, is directly transferred from the drum to the sheet. In the case of full-color image formation, toner images of different colors formed on a plurality of image carriers are sequentially transferred to an intermediate image transfer body one above the other to form a composite color image (primary image transfer), and then the composite color image is transferred to a sheet (secondary image transfer).
The intermediate image transfer body is usually implemented as a belt or a drum. As for a belt, Japanese Patent Laid-Open Publication Nos. 5-270686 and 8-152790, for example, propose to sequentially transfer toner images of different colors to one surface of a sheet being conveyed by a belt via consecutive image formation stations while electrostatically adhering to the belt. Japanese Patent Laid-Open Publication No. 2001-109325 discloses an image forming apparatus constructed to circulate a sheet via consecutive image forming stations by use of a belt in order to form toner images on both surfaces of the sheet.
An image forming apparatus of the type including image forming stations arranged side by side along a belt is generally referred to as a tandem, four-color image forming apparatus. The image forming stations use color toners complementary to separated colors, i.e., red, green and blue and black toner. A problem with this type of image forming apparatus is that color shift occurs if the image transfer start position differs from one image forming station to another image forming station. One of various causes of color shift is the variation of the moving speed of the belt which is, in turn, ascribable to the variation of mechanical characteristics of the belt, particularly the variation of the dimension of the belt ascribable to stretch caused by thermal expansion.
More specifically, a belt is passed over a plurality of metallic rollers and caused to turn thereby. When the belt stretches due to thermal expansion ascribable to heat accumulation, the amount of movement of the belt varies in accordance with the stretch with the result that the moving speed varies for a unit time with respect to a preselected distance.
Today, to meet the increasing demand for the size reduction of an image forming apparatus, when the image forming stations are arranged side by side along the belt, the distance between nearby image forming stations is decreasing. In addition, the distance between a fixing unit configured to fix a toner image on a sheet and the downstream end of a path along which the belt conveys the sheet is decreasing for the same purpose. It is therefore likely that the belt is heated and caused to expand by the fixing unit. Particularly, among rollers over which the belt is passed, a roller adjacent to the fixing unit transfers heat to the belt more than the others due to a material constituting it, aggravating the thermal expansion of the belt.
While a fixing member included in the fixing unit is constantly operated to maintain its surface at preselected temperature, the belt is sometimes brought to a halt when not conveying a sheet. When the belt is held in a halt, part of the belt adjacent to the fixing thermally expands more than the other part. Consequently, after the halt, the belt again starts moving at speed different from expected speed due to stretch ascribable to thermal expansion. This causes the transfer position of an image of the first color and the transfer positions of images of the second and successive colors to be shifted from each other, resulting in color shift. Further, in the case of a monochromatic image, a black image is enlarged in the subscanning direction and becomes defective.
Even when the belt is in movement, part of the belt passed over a roller adjacent to a heat source is apt to thermally expand due to heat transferred via the roller. The moving speed of the belt therefore varies not only at the time of resumption of movement but also during image forming operation, preventing images of different colors from being transferred in accurate register.
To protect the belt from excessive temperature elevation, Japanese Patent Laid-Open Publication No. 2001-296755, for example, proposes to use a heat pipe as the roller adjacent to the fixing unit or to use an exhaust fan for exhausting air around the belt or a cooling fan for cooling the belt. However, such a mechanism for forcibly cooling the belt and roller over which it is passed needs a sophisticated, bulky configuration as well as special control, resulting in an increase in size and cost. Moreover, some lag exists between the time when the heat pipe starts cooling or cooling air starts being fed and the time when the temperature of the belt actually drops, extending a period of time up to the resumption of movement of the belt, i.e., image transfer.